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Wednesday, March 30, 2016

What is the Autocross "PAX" Time All About?

Finish Line Timer (the little yellow thing)

Autocross competition is all about time, and trying to drive the course in as little time as possible. To measure the time to the thousandth of a second, electronic timers are used at the start line and the finish line. The actual time it takes to drive the course is referred to as the RAW time, and for drivers of identical cars it's a great way to see who is the fastest. However, with a wide variety of cars in the same event, in a lot of different classes, it wouldn't be very fair to have a 500 horsepower Corvette compare their time with my 4 cylinder Camry would it? Well of course not, so if you want to get me a Corvette for my birthday, thanks for solving this problem for me! But to compare different classes of cars, we can take the RAW time and then multiply it with a PAX Index factor to get a reasonable comparison of who was the fastest driver.

Start Line Timer, with a starting light on the white board

Sure, if I'm driving the big horsepower car that's modified and set up to be an autocross monster (say an all wheel drive high powered car of some sort that rhymes with Subaru WRX) I'll happily remind you that my RAW time was faster, but that faster time might mostly be due to the car being quicker than the 4 cylinder Camry that is NOT set up for hard driving (if you count things like suspension tuning, racing tires, etc.).

Computers track the timing for all drivers

So how does this PAX thing work? Hey, we've got something to show you! Let's stick with the many classes that are called "Street", which means these cars have very little modifications from stock. Our NMS team drives their Mazda, FIAT, and Ford in Street classes, so we can talk about these cars as examples.

CLASS

PAX INDEX

CAR

RAW TIME

PAX TIME

Super Street

0.835

Porsche Boxster

60

50.1

A Street

0.833

B Street

0.826

C Street

0.819

D Street

0.811

E Street

0.807

99 Mazda Miata

60

48.42

F Street

0.814

G Street

0.806

13 FIAT Abarth

60

48.36

H Street

0.798

15 Ford Fiesta ST

60

47.88

For this example, let's say that a Porsche Boxster and the three NMS autocross cars managed to run the same event and all were timed in exactly 60.00 seconds. To compare times with these different cars, we could multiply each of their times by the appropriate PAX Index for their class, and then compare the new PAX times to see who really was the fastest.

NMS G Street FIAT Abarth

The higher performance Porsche has the largest index multiplier, so they end up with the slowest PAX time! The Pax Index between ES and GS is almost the same, but since the GS class gets a more favorable multiplier (with a lower number), then the 60 second run for the FIAT (48.36) beats the 60 second run for the Miata (48.42). However, the Fiesta in HS has the best PAX Index, so would win our 60 second RAW time tie with a Pax time of 47.88, a good half second faster than the FIAT.
Remember that all four cars drove the course in the same time, but to do so in a Fiesta compared to the Porsche means that Fiesta driver was a much faster driver, in this case by over 2 seconds!

NMS E Street Mazda Miata

For the autocross driver then, they're looking at both their RAW time to see how they do against similar cars at any one event, and also looking at the PAX time to see how they fare against all drivers in all classes. I think that's one thing that makes autocross more interesting, since there are multiple ways to compare your performance with other drivers (or talk trash if that's your thing!). If you want to learn more info on the PAX Index, check out this link: 2016 PAX/RTP INDEX LINK

NMS H Street Ford Fiesta ST

Then just for fun, I'll point out that there is a class of highly modified quick cars, titled A Modified, that has a PAX Index of 1.00, so they get NO advantage with PAX times, which of course means that they are the very quickest of cars.